Gasification is a process by which either a solid or liquid carbonaceous material (e.g., biomass, coal, petroleum), containing mostly chemically bound carbon, hydrogen, and oxygen, and a variety of inorganic and organic constituents, is reacted with air, oxygen, and/or steam. Sufficient energy is provided to produce a primary gaseous product comprising mostly of CO, H2, CO2, H2O(g), and light hydrocarbons laced with volatile and condensable organic and inorganic compounds (e.g., tars). When gasified with steam and/or oxygen, biomass will produce a product gas, sometimes referred to as “synthesis gas” or “syngas” that is rich in CO and H2. Synthesis gas can then be catalytically converted to produce high-value fuels and chemicals.
Unless the raw product gas is combusted immediately following production, it is generally cooled, filtered, and scrubbed with water or a process-derived liquid to remove condensables and any carry-over particles. Alternatively, the raw gas can undergo either medium-temperature (350° C. to 400° C.) or high-temperature (up to gasifier exit temperatures) gas cleaning to provide a fuel gas that can be used in a variety of energy conversion devices, including internal combustion engines, gas turbines, and fuel cells.
A major barrier to the energy efficient and environmentally benign utilization of biomass by gasification is the “clean-up” of the product gas. Unless the gas can be used hot, for example in an adjacent boiler, removal of condensable tars from the product gas is usually necessary. For example, one of the most efficient and cleanest ways to use biomass to generate power to date is to use the product gas in a gas combustion turbine; however, this application requires that essentially all condensable tars are removed from the product gas. Another promising application of a product gas derived from biomass gasification is for the synthesis of a wide variety of liquid fuels, including ethanol and higher alcohols. Similar to the gas turbine applications, the synthesis of alcohols from a biomass gasification product gas also demands the removal of tars and, if possible, even non-condensable hydrocarbons from the product gas.
Several approaches have been tested for the removal of tar from the product gas of biomass gasification, such as catalytic and non-catalytic cracking of tar, dry scrubbing with activated carbon and filters, wet scrubbing, steam reformation of tar, and partial oxidation of biomass directly to produce synthesis gas. However, according to a study published by the National Renewable Energy Laboratory, currently available technologies for tar removal do not meet the needs of the industry in terms of cost, performance, and environmental considerations (T. A. Milne, et al., Biomass Gasification Tars, Their Nature, Formation, and Conversion, NREL/TP 570-25357, November 1998).
Accordingly, there is a need for systems and methods for the reduction in the level of tar or destruction of tar from synthesis gas. It is to the provision of such systems and methods for the reduction in the level of tar or destruction of tar from synthesis gas that the various embodiments of the present invention are primarily directed.